An overview of the Wyoming Weather Modification Pilot Project (WWMPP) is presented. This project, funded by the State of Wyoming, is designed to evaluate the effectiveness of cloud seeding with silver iodide in the Medicine Bow and Sierra Madre Ranges of south-central Wyoming. The statistical evaluation is based on a randomized crossover design for the two barriers. The description of the experimental design includes the rationale behind the design choice, the criteria for case selection, facilities for operations and evaluation, and the statistical analysis approach. Initial estimates of the number of cases needed for statistical significance used historical Snow Telemetry (SNOTEL) data (1987–2006), prior to the beginning of the randomized seeding experiment. Refined estimates were calculated using high-resolution precipitation data collected during the initial seasons of the project (2007–10). Comparing the sample size estimates from these two data sources, the initial estimates are reduced to 236 (110) for detecting a 10% (15%) change. The sample size estimates are highly dependent on the assumed effect of seeding, on the correlations between the two target barriers and between the target and control sites, and on the variance of the response variable, namely precipitation. In addition to the statistical experiment, a wide range of physical studies and ancillary analyses are being planned and conducted.
The 2004 National Academies report on the future of weather modification research is examined by a panel representing the NRC and the Weather Modification Association.
A numerical modeling study has been conducted to explore the ability of the Weather Research and Forecasting (WRF) model-based large-eddy simulation (LES) with 100-m grid spacing to reproduce silver iodide (AgI) particle dispersion by comparing the model results with measurements made on 16 February 2011 over the Medicine Bow Mountains in Wyoming. Xue et al.'s recently developed AgI cloud-seeding parameterization was applied in this study to simulate AgI release from ground-based generators. Qualitative and quantitative comparisons between the LES results and observed AgI concentrations were conducted. Analyses of turbulent kinetic energy (TKE) features within the planetary boundary layer (PBL) and comparisons between the 100-m LES and simulations with 500-m grid spacing were performed as well. The results showed the following: 1) Despite the moist bias close to the ground and above 4 km AGL, the LES with 100-m grid spacing captured the essential environmental conditions except for a slightly more stable PBL relative to the observed soundings. 2) Wind shear is the dominant TKE production mechanism in wintertime PBL over complex terrain and generates a PBL of about 1000-m depth. The terrain-induced turbulent eddies are primarily responsible for the vertical dispersion of AgI particles. 3) The LES-simulated AgI plumes were shallow and narrow, in agreement with observations. The LES overestimated AgI concentrations close to the ground, which is consistent with the higher static stability in the model than is observed. 4) Non-LES simulations using PBL schemes had difficulty in capturing the shear-dominant turbulent PBL structure over complex terrain in wintertime. Therefore, LES of wintertime orographic clouds with grid spacing close to 500 m or finer are recommended.
The ASCII campaign was conducted in early 2012 over the Sierra Madre, a Continental Divide range in Wyoming, to examine cloud microphysical changes resulting from the ground-based injection of silver iodide (AgI) into shallow orographic clouds. The campaign included a dual-polarization Doppler-On-Wheels radar on the mountain pass, the University of Wyoming King Air aircraft with profiling mm-wave radar and polarization backscatter lidar, two Ka-band profiling radars, one upstream of the AgI generators, and one downstream, and a number of instruments on the ground to characterize the cloud liquid water content and the in situ snow particle characteristics. The experimental design supported both simultaneous (upstream/ downstream) and non-simultaneous (before/ during) comparisons, in order to tease out the impact of glaciogenic seeding on cloud and precipitation. Unfortunately, suitably cold cloud base temperatures rarely occurred during the campaign. Nevertheless, profiling radar data collected on flights with a no-seeding period followed by a seeding period, as well as simultaneous ground-based profiling radar data, indicate that under suitable conditions AgI seeding can significantly increase radar reflectivity in the boundary layer, and thus snowfall rate near the ground.
The North Dakota Thunderstorm Project was conducted in the Bismarck, North Dakota, area from 12 June through 22 July 1989. The project deployed Doppler radars, cloud physics aircraft, and supporting instrumentation to study a variety of aspects of convective clouds. These included transport and dispersion; entrainment; cloud-ice initiation and evolution; storm structure, dynamics, and kinematics; atmospheric chemistry; and electrification.Of primary interest were tracer experiments that identified and tracked specific regions within evolving clouds as a means of investigating the transport, dispersion, and activation of ice-nucleating agents as well as studying basic transport and entrainment processes. Tracers included sulfur hexafluoride (SF 6 ), carbon monoxide, ozone, radar chaff, and silver iodide.Doppler radars were used to perform studies of all scales of convection, from first-echo cases to a mesoscale convective system. An especially interesting dual-Doppler study of two splitting thunderstorms has resulted.The objectives of the various project experiments and the specific facilities employed are described. Project highlights and some preliminary results are also presented.
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